The present invention concerns a process for preparing a pigment which is used for a coated paper. More particularly, this invention relates to an improvement of a method for preparing a pigment called `satin white`, which consists essentially of calcium sulfate and aluminum hydroxide.
In manufacturing coated papers, numerous pigment combinations have been employed to improve the productivity and the printability of paper with good sheet gloss at a lower cost.
As a result of the recent increase in the speed of producing paper, and the ongoing need to improve productivity and printability, while reducing cost, there is a need to improve the fluidity of slurry and color (a coating mixture of slurry and adhesive).
Satin white is currently used as a pigment for coated paper because it provides coated paper with an excellent glossy white color and opacity, and improved dryness of printing ink and printability by making the coated surface alkaline. Further, satin white can be treated with calender treatment for an excellent sheet gloss.
U.S. Pat. No. 3,713,881 (T. Akiyama et al.) describes an aqueous coating composition essentially containing water-soluble modified polyvinyl alcohol as a coating binder having one kind, or plural kinds, of substituent groups selected from cyanoethyl, etc.
U.S. Pat. No. 4,952,278 (P. E. Gregory & K. D. Vinson) describes a paper structure having both high opacity and improved tensile strength through the incorporation of extended fiber and an opacifying mineral pigment, such as titanium dioxide.
In U.S. Pat. No. 3,660,131 (M. H. Murray & E. E. Brodhag), satin white coating pigment of improved uniformity, which yields a paper coating having improved physical properties, is prepared by slowly adding an aqueous solution of alum or aluminum sulfate to a stirred aqueous dispersion containing slaked lime or calcium hydroxide and sodium hydroxide.
U.S. Pat. No. 3,798,047 (P. Delfosse) describes a pigment comprising natural calcium carbonate and satin white.
U.S. Pat. No. 3,854,971 (R. F. Conley et al.) describes a method of making a paper coating pigment by forming a satin white in aqueous suspension, adding thereto a kaolinite, and removing a major portion of water from the aqueous suspension to form a free flowing powder.
U.S. Pat. No. 4,241,143 (Y. Ashie & Y. Nakamura) describes a cast coated paper produced by applying an aqueous coating color, which contains satin white, and a polymer latex having a Tg.
U.S. Pat. No. 5,030,325 (S. Saji et al.) describes a method of manufacturing gloss coated paper by applying a coating, composition, whose pigment contains satin white and an organic pigment, and adhesive on base paper.
In the prior art, two kinds of satin white have been known, i.e. a large pillar-shaped particle with 2.about.10 .mu.m of mean particle size, and a smaller needle-shaped particle with 0.1.about.2.0 .mu.m of mean particle size. The former can be obtained by batch-type addition of aluminum sulfate (Al.sub.2 (SO.sub.4).sub.3) to an aqueous suspension of calcium hydroxide (a milk of lime), and the latter by continuous addition of aluminum sulfate thereto.
Though both kinds of particles show excellent whiteness and printing gloss compared to other pigments, it has been found that a slurry of the pillar-shaped particle in high concentration is not suitable for a high speed coating process, as it is dilatant due to its large particle size. However, the needle-shaped particles in the form of color in high concentration can be used in a high speed coating process due to its superior fluidity.
Nowadays, a more improved satin white pigment which provides more uniform particle distribution, improved thixotropy, and excellent printability is required.